Friction stir welding has been used in industry for many years, and especially in welding of metallic materials such as aluminum, brass and bronze. It is a solid state process that involves local plastic deformation via friction heat provided by a rotating and moving friction stir welding probe forced to move along the joint to be welded. The friction heat from the contact between the probe and the metallic material in the joint makes the metallic material stirable, and the rotation and movement of the probe along the joint results in a weld of stirred material.
Friction stir welding is a technique that has a potential to yield large economical benefits compared to traditional laser welding or fusion welding which involves rapid solidification of molten material. Advantages with friction stir welding are the limited area that is heated and also that the resulting weld is often sufficiently smooth to eliminate a subsequent polishing step.
Friction stir welding of steel can provide a weld with less pores, less carbon diffusion and a higher strength compared to a traditional arc welding or laser beam welding.
The condition for the friction stir welding probe during a welding process of steel is highly demanding. Friction stir welding involves thermal cycling at high temperatures. The temperature in the weld is probably about 800-1000° C., and the mechanical strength of the probe must be high at this high temperature. If the mechanical strength at high temperature is insufficient, the probe will suffer from oxidation, wear and collapse or break.
US2010/0258612A1 discloses a friction stir welding tool for welding steel made of hard metal partly coated with one or more layers.
There is a need for further improvements in the field of friction stir welding tools. A friction stir welding tool should not be too expensive, have a long and predictable lifetime and comprise high strength and wear resistance at high temperatures.